1. Field of the Invention
The invention relates to a refrigeration plant having a cold head which is thermally coupled, via a system of lines for a refrigerant which circulates in accordance with a thermosyphon effect, to parts of an installation which are to be cooled. A corresponding refrigeration installation is also given by WO 00/13296 A.
2. Description of the Related Art
In addition to metallic superconductor materials, such as NbTi or Nb3Sn, which have long been known and have very low critical temperatures Tc and are therefore also referred to as low-Tc superconductor materials or LTS materials, metal-oxide superconductor materials with critical temperatures Tc of over 77 K have been known since 1987. The latter materials are also known as high-Tc superconductor materials or HTS materials.
Attempts have also been made to create superconducting windings with conductors using HTS materials of this type. However, the conductors of such windings, on account of their hitherto still relatively low ability to carry current in magnetic fields in particular with inductions in the Tesla range, despite the inherently high critical temperatures of the materials used, are nevertheless frequently held at a temperature level below 77 K, for example between 10 and 50 K, in order in this way to make it possible to carry significant currents at field strengths of several Tesla.
Refrigeration units in the form of cryogenic coolers with a closed He compressed gas circuit are preferentially used to cool windings with HTS conductors within the abovementioned temperature range. Cryogenic coolers of this type are in particular in the form of Gifford-McMahon or Stirling type, or in the form of what are known as pulse tube coolers. Corresponding refrigeration units moreover have the advantage that the refrigeration capacity is available virtually at the push of a button, and there is no need for the user to handle cryogenic liquids. When refrigeration units of this type are used, a superconducting device, such as a magnet coil or a transformer winding, is cooled only indirectly by heat conduction to a cold head of a refrigerator (cf. for example also “Proc. 16th Int. Cryog. Engng. Conf. [ICEC 16]”, Kitakyushu, J P, 20-24.05.1996, Verlag Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also provided for the superconducting rotor of an electrical machine which is disclosed by the WO-A document cited in the introduction. The rotor includes a winding composed of HTS conductors, which can be kept at a desired operating temperature of well below 77 K by a refrigeration unit configured as a cryogenic cooler. The refrigeration unit includes a cold head located outside the rotor. The colder side of this cold head is thermally coupled to the winding using neon as refrigerant, which circulates in a system of lines, which includes parts that project into the rotor as far as the winding, using a thermosyphon effect. In the event of a fault in the refrigeration unit, in particular its cold head, or in the event of the latter having to be repaired or exchanged, however, it is almost impossible to maintain the operating state of the winding which is to be cooled.
Furthermore, EP 0 696 380 B1 discloses a superconducting magnet of an MRI installation which, to cool its superconducting winding, has a refrigeration plant which includes two refrigeration units in the form of cryogenic coolers. The two cold heads of these cryogenic coolers are thermally coupled to a solid heat conduction body which is thermally conductively connected to parts of the winding that are to be cooled. The cold heads of the two cryogenic coolers are each accommodated in a dedicated vacuum space, so that during operation of one cryogenic cooler the second can be switched off and/or exchanged. On account of the connection of a plurality of cold heads with good thermal conductivity to form the same parts that are to be cooled, however, additional thermal conduction losses caused by a cold head which may be switched off generally have to be accepted.